Minuscule risk? The amount of zinc absorbed into the bloodstream from nanoparticle sunscreen is a tiny fraction of that naturally present in the body, according to an Australian study.

CSIRO researchers, led by Dr Maxine McCall, conducted a trial in a typical beach setting where volunteers applied a controlled amount of zinc oxide 'nano-sunscreen' to their skin over five days.

The sunscreen was designed so the zinc could be distinguished from natural levels of zinc, which is considered an essential mineral in the human body.

"The levels we found in blood were very small," says McCall, a research consultant on nano-safety at CSIRO. "After applications over five days, the levels of the tracer zinc in the blood were one thousandth of what is the naturally-occurring total zinc levels in the blood."

However the study was unable to determine whether the zinc in the blood stream was in nanoparticulate form or dissolved. Study samples had to be treated before analysis, which meant any nanoparticles in the blood and urine samples would have dissolved beforehand.

"If it's dissolved zinc, there's absolutely no issue because even if more comes in over a lifetime of use, zinc levels are well controlled in the human body so it would be urinated out or excreted or stored in proteins as dissolved zinc," McCall says.

"If it's nanoparticles of zinc oxide coming in, we don't know what the consequences would be."

The study found that women had higher levels of zinc in their blood than men.

Possible explanations included gender differences in skin chemistry and response to ultraviolet light, or differences in the amount of hair on the back where the sunscreen was applied.

They also found evidence that the zinc is retained in the blood as levels of the zinc continued to rise for up to six days after final application of the nano-sunscreen. This suggested the zinc was being stored somewhere in the body, either in dissolved form bound to proteins or as nanoparticles in hair follicles on the skin.

Coordinator of the NanoSafe Australia network Associate Professor Paul Wright from RMIT University says people need to weigh up the known risk of excessive UV light exposure causing skin cancer, and the perceived risk of using nano-sunscreens such as zinc oxide, which was the most effective, broad spectrum sunscreen.